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Subject[PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in rcu_segcblist structure
Date
Upcoming ->nocb_lock contention-reduction work requires that the
rcu_segcblist structure's ->len field be concurrently manipulated,
but only if there are no-CBs CPUs in the kernel. This commit
therefore makes this ->len field be an atomic_long_t, but only
in CONFIG_RCU_NOCB_CPU=y kernels.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
include/linux/rcu_segcblist.h | 4 ++
kernel/rcu/rcu_segcblist.c | 86 ++++++++++++++++++++++++++++++++---
kernel/rcu/rcu_segcblist.h | 12 ++++-
3 files changed, 94 insertions(+), 8 deletions(-)

diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index 82977726da29..38e4b811af9b 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -65,7 +65,11 @@ struct rcu_segcblist {
struct rcu_head *head;
struct rcu_head **tails[RCU_CBLIST_NSEGS];
unsigned long gp_seq[RCU_CBLIST_NSEGS];
+#ifdef CONFIG_RCU_NOCB_CPU
+ atomic_long_t len;
+#else
long len;
+#endif
long len_lazy;
u8 enabled;
u8 offloaded;
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 92968b856593..ff431cc83037 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -23,6 +23,19 @@ void rcu_cblist_init(struct rcu_cblist *rclp)
rclp->len_lazy = 0;
}

+/*
+ * Enqueue an rcu_head structure onto the specified callback list.
+ * This function assumes that the callback is non-lazy because it
+ * is intended for use by no-CBs CPUs, which do not distinguish
+ * between lazy and non-lazy RCU callbacks.
+ */
+void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
+{
+ *rclp->tail = rhp;
+ rclp->tail = &rhp->next;
+ WRITE_ONCE(rclp->len, rclp->len + 1);
+}
+
/*
* Dequeue the oldest rcu_head structure from the specified callback
* list. This function assumes that the callback is non-lazy, but
@@ -44,6 +57,67 @@ struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp)
return rhp;
}

+/* Set the length of an rcu_segcblist structure. */
+void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+ atomic_long_set(&rsclp->len, v);
+#else
+ WRITE_ONCE(rsclp->len, v);
+#endif
+}
+
+/*
+ * Increase the numeric length of an rcu_segcblist structure by the
+ * specified amount, which can be negative. This can cause the ->len
+ * field to disagree with the actual number of callbacks on the structure.
+ * This increase is fully ordered with respect to the callers accesses
+ * both before and after.
+ */
+void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+ smp_mb__before_atomic(); /* Up to the caller! */
+ atomic_long_add(v, &rsclp->len);
+ smp_mb__after_atomic(); /* Up to the caller! */
+#else
+ smp_mb(); /* Up to the caller! */
+ WRITE_ONCE(rsclp->len, rsclp->len + v);
+ smp_mb(); /* Up to the caller! */
+#endif
+}
+
+/*
+ * Increase the numeric length of an rcu_segcblist structure by one.
+ * This can cause the ->len field to disagree with the actual number of
+ * callbacks on the structure. This increase is fully ordered with respect
+ * to the callers accesses both before and after.
+ */
+void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp)
+{
+ rcu_segcblist_add_len(rsclp, 1);
+}
+
+/*
+ * Exchange the numeric length of the specified rcu_segcblist structure
+ * with the specified value. This can cause the ->len field to disagree
+ * with the actual number of callbacks on the structure. This exchange is
+ * fully ordered with respect to the callers accesses both before and after.
+ */
+long rcu_segcblist_xchg_len(struct rcu_segcblist *rsclp, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+ return atomic_long_xchg(&rsclp->len, v);
+#else
+ long ret = rsclp->len;
+
+ smp_mb(); /* Up to the caller! */
+ WRITE_ONCE(rsclp->len, v);
+ smp_mb(); /* Up to the caller! */
+ return ret;
+#endif
+}
+
/*
* Initialize an rcu_segcblist structure.
*/
@@ -56,7 +130,7 @@ void rcu_segcblist_init(struct rcu_segcblist *rsclp)
rsclp->head = NULL;
for (i = 0; i < RCU_CBLIST_NSEGS; i++)
rsclp->tails[i] = &rsclp->head;
- rsclp->len = 0;
+ rcu_segcblist_set_len(rsclp, 0);
rsclp->len_lazy = 0;
rsclp->enabled = 1;
}
@@ -151,7 +225,7 @@ bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp)
void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
struct rcu_head *rhp, bool lazy)
{
- WRITE_ONCE(rsclp->len, rsclp->len + 1); /* ->len sampled locklessly. */
+ rcu_segcblist_inc_len(rsclp);
if (lazy)
rsclp->len_lazy++;
smp_mb(); /* Ensure counts are updated before callback is enqueued. */
@@ -177,7 +251,7 @@ bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,

if (rcu_segcblist_n_cbs(rsclp) == 0)
return false;
- WRITE_ONCE(rsclp->len, rsclp->len + 1);
+ rcu_segcblist_inc_len(rsclp);
if (lazy)
rsclp->len_lazy++;
smp_mb(); /* Ensure counts are updated before callback is entrained. */
@@ -204,9 +278,8 @@ void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
rclp->len_lazy += rsclp->len_lazy;
- rclp->len += rsclp->len;
rsclp->len_lazy = 0;
- WRITE_ONCE(rsclp->len, 0); /* ->len sampled locklessly. */
+ rclp->len = rcu_segcblist_xchg_len(rsclp, 0);
}

/*
@@ -259,8 +332,7 @@ void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
rsclp->len_lazy += rclp->len_lazy;
- /* ->len sampled locklessly. */
- WRITE_ONCE(rsclp->len, rsclp->len + rclp->len);
+ rcu_segcblist_add_len(rsclp, rclp->len);
rclp->len_lazy = 0;
rclp->len = 0;
}
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index db38f0a512c4..1ff996647d3c 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -9,6 +9,12 @@

#include <linux/rcu_segcblist.h>

+/* Return number of callbacks in the specified callback list. */
+static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
+{
+ return READ_ONCE(rclp->len);
+}
+
/*
* Account for the fact that a previously dequeued callback turned out
* to be marked as lazy.
@@ -42,7 +48,11 @@ static inline bool rcu_segcblist_empty(struct rcu_segcblist *rsclp)
/* Return number of callbacks in segmented callback list. */
static inline long rcu_segcblist_n_cbs(struct rcu_segcblist *rsclp)
{
+#ifdef CONFIG_RCU_NOCB_CPU
+ return atomic_long_read(&rsclp->len);
+#else
return READ_ONCE(rsclp->len);
+#endif
}

/* Return number of lazy callbacks in segmented callback list. */
@@ -54,7 +64,7 @@ static inline long rcu_segcblist_n_lazy_cbs(struct rcu_segcblist *rsclp)
/* Return number of lazy callbacks in segmented callback list. */
static inline long rcu_segcblist_n_nonlazy_cbs(struct rcu_segcblist *rsclp)
{
- return rsclp->len - rsclp->len_lazy;
+ return rcu_segcblist_n_cbs(rsclp) - rsclp->len_lazy;
}

/*
--
2.17.1
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